1. Field of the Invention
The present invention relates to an apparatus for directly bonding thermoplastic members, such as a thermoplastic pad to a pile carpet, using radio-frequency electromagnetic energy (RF energy).
2. Description of the Prior Art
The presence of wear-resistant pads on carpeted floor areas of an automobile interior is familiar to all drivers. In most cases such pads are fabricated of a thermoplastic sheet material having a predetermined peripheral shape. Typically, the pad is fabricated from a sheet of vinyl, such as polyvinyl chloride. The thermoplastic pad is either adhesively or directly bonded to the carpet. In typical adhesive bonding a thermoplastic adhesive is applied between the pad and the carpet, and radio-frequency electromagnetic energy (RF energy) or heat is used to melt the adhesive and effect the bond between the pad and the carpet. In an adhesive bonding situation overheating or burning is not usually a problem because the melting point of the thermoplastic adhesive is sufficiently lower than that of the materials of the carpet pile or pad. In direct bonding RF energy is applied to the pad and to the carpet to effect the bond therebetween. Direct bonding using RF energy is also known as "dielectric bonding". Such direct bonding of the pad to the carpet is preferred since no adhesive is required and since the bonding cycle time is reduced.
However, as will be fully explained herein, in some instances, especially in production environments, direct bonding of a pad to a carpet is difficult to achieve without overheating or discoloring of the carpet pile. This overheating or discoloring occurs due to increased field intensities caused by edge effects associated with the geometry of the electrodes of the bonding apparatus. The overheating or discoloring is exacerbated by the particular temperature dependence of the susceptibility of the carpet material to radio frequency energy. The term "susceptibility" as used herein means the ability of a material to convert electric field intensity to heat.
A direct bonding apparatus generally indicated by reference character 10 that is used in the prior art for applying RF energy to effect a bond between a first thermoplastic member, such as a thermoplastic pad P, to a first surface of a second thermoplastic member, such as a thermoplastic carpet C, is shown in FIGS. 1A and 1B. A finished automotive carpet C having the pad P bonded thereto is shown in FIGS. 2A and 2B. As suggested in FIG. 2A the pad P has a predetermined peripheral shape depending upon the region of the automobile interior on which it is disposed. The pad P has a embossed pattern formed therein. The pattern has a flat peripheral border region B surrounding a plurality of raised areas (R) with flat regions (F) therebetween.
The bonding apparatus 10 includes a first, die, electrode 14 and a second, backing, electrode 22. The electrodes 14, 22 are confrontationally arranged with respect to each other. The die electrode 14 includes a generally planar mounting portion 16 to which is attached a configured die member 18. Both the die electrode 14 (formed of mounting portion 16 and die member 18) as well as the backing electrode 22 are fabricated from an electrically conductive material, such as metal or a conductive composite.
The die member 18 has a predetermined height dimension 18H and has a peripheral shape that corresponds to the peripheral shape of the pad P to be bonded in the apparatus 10. In addition, the operative surface of the die member 18 has one or more relief features 18F, the edges of which are defined by depressions 18D. The relief features 18F of the die member 18 correspond to the border region B and the flat regions F on the pad P, while the depressions 18D on the die member 18 correspond to the raised areas R on the pad P. The width dimensions of the depressions 18D (such as dimensions 18W1, 18W2) and the depth dimension of the depressions 18D (such as the dimension 18E) and thus, the width and height dimensions of the raised areas R, are dictated in accordance with the design of the pattern to be imparted into the pad P. Typically, the depth dimension 18E is on the order of several millimeters.
In the prior art apparatus 10 the backing electrode 22 is generally planar. The electrodes 14, 22 are mounted within a framework 28 of a press 26 (FIG. 1A). Although the electrodes are shown as horizontally arranged with the die electrode 14 being located beneath the bonding electrode 22, it should be understood throughout this application that any convenient orientation of the electrodes 14, 22 with respect to the framework 28 and to each other may be used. The press 26 includes an actuator 30 operable to move the electrodes 14, 22 relative to each other (as suggested by the directional arrow 34) from an open position (shown FIG. 1A) to a closed position (suggested in FIG. 1C). Both electrodes 14, 22 are electrically connected to a suitable source 36 of high voltage RF energy. Typically, the source 36 outputs a radio frequency signal in the range from about one megahertz (1 MHz.) to one-hundred megahertz (100 MHz.) at a voltage in the range from about three thousand volts (3 kV) to about ten thousand volts (10 kV). Typical power levels of radio frequency bonding apparatus is in the range of five kilowatts (5 kW) to one hundred kilowatts (100 kW).
To bond a pad P to the carpet C the pad P is placed on the configured die member 18 of the die electrode 14, as is seen in FIG. 1A. The carpet C is thereafter placed with its pile surface S in contact with the undersurface of the pad P. The surface of the backing B of the carpet C is presented toward the backing electrode 22. A resilient buffer layer 38, typically fabricated of a material (such as silicone rubber) having a low susceptibility to RF energy, is interposed between the surface of the backing B of the carpet C and the backing electrode 22.
After the materials have been layered into their relative positions as described and illustrated in FIG. 1A a bonding cycle is initiated. The actuator 30 is asserted to move the backing electrode 22 toward the die electrode 14 to clamp the layered materials with a predetermined clamping pressure. Clamping pressure is usually specified in terms of the resulting gap G (FIG. 1C) defined between the raised area 18F of the die electrode 14 and the backing electrode 22. In practice the gap G is limited by a physical stop (not shown) in the press mechanism.
With the materials clamped the source 36 is activated and RF energy is applied between the electrodes 14, 22 to the layered materials clamped therebetween. The RF energy is applied at a predetermined voltage (typically, on the order of three thousand to ten thousand volts) for a predetermined period of time, termed the "heat cycle" (typically, on the order of five to twenty seconds), to heat the materials of the pad P and the pile S of the carpet C. The source 36 is deactivated and the materials remain clamped for a second predetermined period of time, termed the "soak cycle" (typically, also on the order of five to twenty seconds) to permit the materials that were heated to cool and the bond between them to set. In a satisfactory bond the material of the pad P is adhered over the entire interfacial area of the border region B and the flat regions F between the pad P and the carpet C, that is, the pad is "fully adhered". Also, in a satisfactory bond, the fully adhered condition is achieved without discoloring or melt-through of the pad P or discoloring or excessive melting of the carpet pile S adjacent to the periphery of the pad P. The range of combinations of voltages and times able to produce a fully adhered pad is termed the "operating window".
It has been found that, especially in a production environment, a conventional bonding apparatus is not able to produce a satisfactory bond between a pad and a carpet when the pile of the carpet is producer-colored nylon having a topical stain-resist material. No matter what voltage and time parameters are chosen for the operating window either (1) the pad P is not fully adhered to the carpet C, (2) discoloring or perforations occur in the pad P adjacent to the corners or edges of the raised areas R, and/or (3) discoloring or melting of the carpet C occurs adjacent to the periphery of the pad P. The inability to achieve a satisfactory bond is believed due the combination of (1) localized increases in electric field intensity due to edge effects resulting from the geometry of the coupled electrodes of the bonding apparatus, (2) the temperature-dependent RF susceptibility characteristic of the material of the carpet, and (3) the RF susceptibility and thermal conductivity characteristics of the carpet backing.
FIG. 1C illustrates the electric field lines (shown as fine lines) between the die electrode and backing electrode in the portion of FIG. 1A enclosed by the dashed box labeled "1C". For clarity of illustration the materials to be bonded that lie within the gap G are not shown. Relatively sharp-edged features are defined along the periphery of the die member 18 as well as along the relief features 18F thereof. The peripheral edges and the relief feature edges, together with the backing electrode 22, produce an electric field pattern in which field lines tend to converge in the vicinity of the edges. The spacing of adjacent field lines in FIG. 1C indicates the field intensity.
FIG. 4 shows a quantitative plot of the square of the field intensity (normalized) versus the relative lateral position of the field lines of FIG. 1C. The abscissa of the plot is relative lateral position, while the ordinate of the plot is the square of the field intensity (normalized to the field intensity between the central region of a relief feature 18F and the backing electrode 22). The plot is taken along a reference line 4--4 that lies a distance above the die member 18 equal to about ten percent of the width of the gap G between the electrodes 18, 22. The reference line 4--4 is selected to approximate the position at which the bond between the pad P and the carpet C occurs. The prior art field intensity is indicated by a dashed line carrying the reference label "Prior Art".
In automotive applications the pile surface S of the carpet C is usually formed from a thermoplastic polymeric material, typically a polyamide such as nylon. Nylon is the preferred due to its wear characteristics. However, nylon (and especially nylon 6,6) differs from other polymeric materials such as polyester and polypropylene used for carpets because the susceptibility of nylon to radio frequency energy increases with increasing temperature. A material having such a temperature dependent susceptibility characteristic is subject to a phenomenon known as "thermal runaway". In a thermal runaway situation the material of the carpet melts so that individual tufts lose their definition. Discoloration may also occur. The pile surface S of the carpet thus takes on an amorphous crusty appearance. The peak in the field intensity associated with a peripheral edge of the die electrode is indicated at reference character 40 in FIG. 4. The peak in the field intensity associated with a relief feature edge of the die electrode is indicated at reference character 42 in FIG. 4. These peak intensities, coupled with the particular temperature-dependent susceptibility characteristic of the nylon, results in a localized overheating which causes thermal runaway in the vicinity of the edges on the die member 18. Discoloration and an amorphous crusty appearance may be manifested adjacent to the periphery of the pad P. Perforations of the pad may occur in interior regions of the pad P due to the combination of thermal runaway of the carpet pile material and RF heating of the pad itself. These perforations usually occur at edges, and especially at corners, of relief features. Depending upon the material used the backing of the carpet may also contribute to the overall overheating problem.
The presence of additives such as colored pigments and/or topical stain-resist materials further increases the temperature dependence of the susceptibility of nylon to radio frequency energy and exacerbates the thermal runaway problem. It is believed that the further increase in temperature dependence is due to the mobility of the ions in the topical stain-resist material.
In view of the foregoing it is believed to be advantageous to provide a bonding apparatus that minimizes the problems of localized excessive heating when bonding a first thermoplastic member to a second thermoplastic member. More particularly, it is believed to be advantageous to provide a bonding apparatus that minimizes the problem of thermal runaway when bonding a thermoplastic pad to a nylon pile carpet.